Vertically stacked photovoltaic and thermal solar cell

ABSTRACT

According to some embodiments, the present invention provides a novel photovoltaic solar cell system from photovoltaic modules that are vertically arrayed in a stack format using thin film semiconductors selected from among organic and inorganic thin film semiconductors. The stack cells may be cells that are produced in a planar manner, then vertically oriented in an angular form, also termed herein tilted, to maximize the light capturing aspects. The use of a stack configuration system as described herein allows for the use of a variety of electrode materials, such as transparent materials or semitransparent metals. Light concentration can be achieved by using fresnel lens, parabolic mirrors or derivatives of such structures. The light capturing can be controlled by being reflected back and forth in the photovoltaic system until significant quantities of the resonant light is absorbed. Light that passes to the very end and can be reflected back through the device by beveling or capping the end of the device with a different refractive index material, or alternatively using a reflective surface. The contacting between stacked cells can be done in series or parallel. According to some embodiments, the present invention uses a concentrator architecture where the light is channeled into the cells that contain thermal fluid channels (using a transparent fluid such as water) to absorb and hence reduce the thermal energy generation.

CROSS-REFERENCES TO RELATED APPLICATIONS

This application claims priority to U.S. provisional application Ser.No. 61/288,632, entitled Vertically Stacked Photovoltaic and ThermalSolar Cell, filed Dec. 21, 2009, which is hereby incorporated herein byreference.

GOVERNMENTAL SPONSORSHIP

The U.S. Government has a paid-up license in this invention and therights in limited circumstances to require the patent owners to licenseothers on reasonable terms as provided for by the terms of grant No.DE-FG36-08G088008 awarded by the U.S. Department of Energy.

BACKGROUND OF THE INVENTION

The present invention relates to a new kind of photovoltaic (PV) solarcell system that combines concentrator techniques and coolant methods tomaximize the electrical and thermal output from photovoltaic (PV)modules that are vertically arrayed in a stack format using thin filmorganic and/or inorganic semiconductors.

DESCRIPTION OF THE RELATED ART

There is currently an international effort to achieve higher performancefrom thin film devices including organic photovoltaic (OPV) devices andIII-V semiconductor inorganic photovoltaic (IPV) such as but not limitedto Copper Indium Gallium Selenide (CIGS). The goal is to produce modulesthat can produce electricity in an effective but cheap manner.

For inorganic photovoltaic (IPV) cells, to this day thin film solarcells made of III-V semiconductor compounds exhibit the leading energyconversion efficiencies. In 2008, a team at the National RenewableEnergy Laboratory (NREL) achieved 19.9% efficiency in Copper IndiumGallium Selenide (CIGS) solar cells. Besides the potential for highefficiency, III-V semiconductor compound materials also have advantagesincluding the bandgap tunability by elemental compositions, higherphoton absorption by the direct bandgap energies and smaller thermaldegradation than silicon solar cells. Prior art shows that efficiencyimprovement can be achieved by multistacking photovoltaic materials ofdifferent bandgap energies to form so-called multijunction or tandemcells. The idea of tandem cells is to absorb the photon energy from thesunlight spectrum more widely and efficiently by taking the advantage ofthe tunability of bandgap energies and lattice constants with thecompositions of III-V semiconductor compounds. So far, the highestefficiency of about 40% has been achieved by triple-junctionInGaP/GaAs/Ge cells developed independently by NREL, Boeing-Spectrolaband Fraunhofer.

Recent progress in making triple-junction InGaP/GaAs/Ge cells showsgreat promise toward the theoretical limit (50.1% efficiency at AM 1.5D,1000 suns). However, the manufacturing of these cells is far away fromtrivial. For example, a series-connected monolithic triple-junction cellhas more than 15 semiconductor layers. Each layer is deposited byepitaxial growth with metal-organic chemical vapor deposition (MOCVD),which tends to require precise lattice matching among these stackedsemiconductor materials. Although efforts to develop next-generationmanufacturing technology to produce 42% efficient III-V triple-junctiontandem concentrator solar cells sponsored by NREL are currentlyunderway, mass production of these cells will tend to require control ofepitaxial growth by large area. Thus, technical challenges in makingtriple-junction InGaP/GaAs/Ge cells remain. Further, the cost ofmanufacturing these tandem cells may never reach a commercially feasiblegoal of 1$/Watt peak.

Recently, a collaborative team based on a Defense Advanced ResearchProjects Agency's (DARPA) program reported a novel cell module designwith sunlight splitting by dichroic filters and located independentlyinto cells with varied bandgap energies. In this architecture, each cellwill receive a fraction of the solar spectrum absorbed most efficientlyand converted into electrical power. This architecture avoids thecurrent-matching issue among subcells and free carrier absorption lossin upper subcells for monolithic devices. They have independently testedthree cells (two double-junctions and one single-junction) with properfilters to mimic spectral incidence to each cell and reported a 42.7%efficiency simply by summing up the efficiencies of the three cells.This shows that with proper optical design of the cell module, thedesign of each individual cell could be much simpler to manufacturewithout sacrificing the total efficiency.

For organic photovoltaic cells, much of this effort centers on thecreation of polymer-nanoparticle blends which allow for efficientdissociation of photo-generated excitons. Device architectures based onthese “bulk-heterojunction” blends have yielded beyond 5% conversion ofincident light to electricity. They have primarily been based onpolythiophene hosts dispersed with C₆₀ (fullerene) conjugates. Thedifficulty arises from the fact that different host materials, theavailable dispersant phase, or nanophase, of the blend may not have theappropriate electronic structure or form the right interface to supportefficient resonant charge transfer. The additional factor of having alow absorption range which is then coupled to the reduced fill factorresults in lower power conversion efficiency.

In a modular form, the limitations in standard device performance caninclude the effect of increased temperatures around the IPV or OPV, andincreasing temperatures have a detrimental effect on the efficiency andlongevity of the solar cells. A problem that is well-known forphotovoltaic devices is the thermal degradation of the cell componentsdue to temperature building caused by exposure to the sun. Over time,thermal degradation tends to affects the longevity of cells. Otherissues also arise in collecting light in regions dominated by diffuselight or weather patterns that disperse direct sunlight into a diffuseformat.

There remain fundamental issues with building successful photovoltaiccells that affect some or all thin film solar cells depending on theactive media and even the use of electrodes and concentrators. Theseissues can include charge carrier transport since polymers, polymercomposites and some inorganic thin films with crystalline defectlimitations can convert some or all resonant light into charge carriers(electrons and holes or excitons), their carrier transport is poor. Withrespect to charge carrier transport, polymers, polymer composites andsome inorganic thin films with crystalline defect limitations canconvert some or all resonant light into charge carriers (electrons andholes or excitons), their carrier transport is poor. The reasonsmanifest themselves in organics since the exciton generated inside thepolymer composite can only travel a very short distance, typically about10 to 20 nm, before being recombined. Secondly, organic basedphotovoltaics possess poor mobilities and conductivities. Inorganiccells such as CdTe:CdSe suffer from defects occurring in the crystallinestructures, causing imitations in the charge carrier transport.Consequently, polymer composite photovoltaic devices can only be madefrom ultra-thin semiconductor films (usually less than 150 nm). Ideally,if the light could be absorbed perfectly, a thin film of 20 nm would beenough, but this would be problematic when controlling pin-holingeffects. The next issue in the challenge of dichotomy is transparencydue to the necessity to have very thin films as a consequence of poorcarrier transport properties, significant light is lost due totransparency. While getting the exciton out from the device may besolved, there is not sufficient material to prevent light from gettingthrough. Finally, the issue is oxidation and water which can affect bothorganic and some inorganic based thin film devices that requirestringently controlled laboratory conditions to minimize oxygen andwater contamination in the polymers composites, which deteriorate deviceperformance over time.

Further there are fundamental issues with electrode materials. In spiteof their cost and limited instability over time, the use of transparentelectrodes such as TO and ITO have been the dominant electrode materialsin existing thin film devices. It is well-known that their performanceare compromised if they are too thin (as their conductivity becomespoor) or too thick (as their transparency greatly diminishes).

SUMMARY

According to some embodiments, the present invention provides a novelphotovoltaic solar cell system that combines concentrator techniques andcoolant methods to maximize the electrical and thermal output fromphotovoltaic modules that are vertically arrayed in a stack format usingthin film semiconductors selected from among organic and inorganic thinfilm semiconductors.

According to some embodiments, the purpose of this invention is toovercome efficiency and manufacturing limitations generally encounteredin existing photovoltaic devices, may they be organic or inorganic solarcells. More specifically, according to some embodiments, the presentinvention addresses fundamental issues with building successfulphotovoltaic cells that affect some or all thin film solar cellsdepending on the active media and even the use of electrodes andconcentrators.

Based on these principles, the stack cells as described according tosome embodiments of the present invention are cells that are produced ina planar manner, then vertically oriented in an angular form, alsotermed herein tilted, to maximize the light capturing aspects.

The use of a stack configuration system as described herein allows forthe use of a variety of electrode materials. In particular, the use of astack configuration system as described herein allows for the use of atransparent electrodes, or semitransparent metals such as but notlimited to Au, Ag (including silver paste). The light capturing can becontrolled by being reflected back and forth until significantquantities of the resonant light is absorbed. Under such circumstances,immediate transparency is not needed as there is a length dimensioninvolved that allows for the light to travel further enabling it to beabsorbed. Light that passes to the very end and can be reflected backthrough the device by beveling or capping the end of the device with adifferent refractive index material, or alternatively using a reflectivesurface.

The devices described herein also addresses thermal degradation.According to some embodiments, the present invention addresses thatissue by having thermal fluid inlets on the base substrate that collectsand dissipates the heat away from the other cell components. This canalso be achieved using the concentrator architecture where the light ischanneled into the cells that contain thermal fluid channels (using atransparent fluid such as water) to absorb and hence reduce the thermalenergy generation. The light concentration can be achieved by usingfresnel lens, parabolic mirrors or derivatives of such structures. Useof concentrator solar cells and lensing mechanisms such as but notlimited to Fresnel lens addresses the issue of collecting light inregions dominated by diffuse light or weather patterns that dispersedirect sunlight into a diffuse format. The contacting between stackedcells can be done in series or parallel.

BRIEF DESCRIPTION OF THE FIGURES

FIG. 1 illustrates a vertical multiple stack photovoltaic device;

FIG. 2A illustrates a basic lensing system;

FIG. 2B illustrates a basic lensing system with thermal parts;

FIG. 3 illustrates a complex lensing system;

FIG. 4A illustrates a rod and plate press system;

FIG. 4B illustrates a rod and bolt press system;

FIG. 5 illustrates an organic photovoltaic (OPV) cell design;

FIG. 6 illustrates an OPV cell design with thermal channels;

FIG. 7 illustrates an inorganic photovoltaic (IPV) cell design;

FIG. 8 illustrates an IPV cell design with the thermal channels;

FIG. 9 illustrates a complex cell design;

FIG. 10 illustrates a type of tilted complex cell design;

FIG. 11 illustrates another type of tilted complex cell design;

FIG. 12 illustrates yet another type of tilted complex cell design;

FIG. 13 illustrates a complex cell design with thermal channels; and

FIG. 14 illustrates another complex cell design with thermal channels;

FIG. 15 illustrates a process for fabricating OPV stacked tandem cells;

FIG. 16 illustrates a side view of OPV stacked tandem cells made by theprocess of FIG. 15;

FIG. 17 is a simplified ray diagram illustrating light passing throughstacked photovoltaic cells;

FIG. 18 is a simplified ray diagram illustrating light passing throughstacked twin photovoltaic (PV) cells;

FIG. 19A illustrates metal strips deposited on a substrate withphotovoltaic (PV) devices;

FIG. 19B shows a side view of stacked photovoltaic (PV) devices shown inFIG. 19A with deposited metal strips;

FIG. 19C illustrates conductive tapes applied on the exposed metalstrips shown in FIG. 19B;

FIG. 19D illustrates metals strips deposited on a flexible substratewith photovoltaic (PV) devices;

FIG. 19E illustrates bending of the flexible substrate shown in FIG. 19Dwithout damaging the connections;

FIG. 19F illustrates stacking of the photovoltaic (PV) devices shown inFIG. 19E; and

FIG. 19G illustrates conductive tapes applied on the exposed metalstrips shown in FIG. 19F.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

In various embodiments, stack photovoltaic devices are described herein.The stack photovoltaic devices include a flat substrate, a firstelectrode layer coating the planar substrate, a continuoussemiconductive (active material—AM) layer deposited above the firstelectrode layer, and a second electrode layer deposited above thecontinuous AM layer. The first electrode layer is at least partiallytransparent to electromagnetic radiation. The continuous AM layer is inelectrical contact with the first electrode layer. The second electrodelayer is in electrical contact with the continuous AM layer. Thecontinuous AM layer absorbs electromagnetic radiation and turns theelectromagnetic radiation into electrical power (voltage and currentgenerated). The continuous AM layer includes at least one semiconductingmaterial, but more generally two in the form of a heterojunction layer,but those continuous lines of different active materials are laid downin series. The devices (as shown in FIG. 1), when formed are thencombined with other planar structures in a series of stacks where thedevices are mounted vertically rather than planar to the incident light.In this way, the light is channeled downwards through the differentlayers until the resonant light has been absorbed and turned intoelectrical power.

FIG. 1 illustrates vertical multiple stack photovoltaic device 100,showing light input 110, fully-reflective Al 120, thin Film organicsemi-conductor (OSC 130), semi-reflective Au 140, substrate 150, andsemi-reflective Al 160. FIG. 1 depicts an example of semitransparentpath system designed for the vertically stacked tandem organicphotovoltaic (OPV) cells.

The substrate slides are made up of but are not limited to amorphoussilicon glass, quartz glass, doped glass that contain fluorescent nanoor micro particles, also/or contain anti-stokes particles.Alternatively, the substrate can be made up of polymers, that can alsocontain anti-stokes with embedded dyes/molecules to shift the absorbedlight from infra-red light to visible light, or UV fluorescent dyes thatcan convert UV light to the visible or near IR.

The initial semiconductor layer generally contains a host p-type polymer(polythiophene, polycarbazole, polybenzothiadiazole, including theircopolymers) and an n-type semiconductor such as but not limited to PCBM(C₆₀ or C₇₀ version) or perylene diimide derivatives. The second layercan contain another polymer (p- or n-type) and a quantum dot or quantumrod (such as, but not limited to, low bandgap nanoparticles such asCu₂S, CdS, CdSe, PbS or PbSe) that absorbs another part of the solarspectrum. The third layer contains another polymer (p- or n-type) and aquantum dot or quantum rod that absorbs another part of the solarspectrum. The purpose of each additional layer is to absorb more of thesolar spectrum that other layers fail to absorb.

The purpose of using metal layers as another option of providing a ‘highwork function’ electrode is to replace transparent electrode Indium TinOxide (ITO). Elements such as Au, Ag, Co, Ni, Pt, Pd and Mo are examplesof high work function metals that may be used as electrodes. However,these electrodes must by definition and need to be semi-transparent.This transparency must range from 10%-70% transparent. The counterelectrode such as Al may be semi-transparent or fully reflective.

A lensing system (as shown in FIGS. 2A and 2B) may also be used toincrease the intensity of light and/or to take in diffuse or lowintensity light. The lenses may comprise of a convex lens, a Fresnellens, or optical combinations to increase the intensity of lightstriking the PV device and/or deflect more incident light (diffuse ordirect light).

FIG. 2A illustrates lensing system 200A, showing lens 210A, and carrierCase 220A. FIG. 2A depicts a basic lensing design where lens 210A, whichmay be a Fresnel lens, sits on top of carrier case 210B which contains aconcentrator.

FIG. 2B illustrates lensing system 200B with thermal parts, showing lens210B, carrier Case 200B, and thermal fluid coolant exit 230. FIG. 2Bdepicts a basic lensing design where lens 210B, which may be a Fresnellens, sits on top of carrier case 210B which contains a concentrator andthermal parts.

Alternatively, a new lensing system that may be used is the bell lenssystem which is used to deflect direct or diffuse light into the device.As shown in FIG. 3, the system consists of a Fresnel lens that focuseslight through the lensing system onto a concave lens which then broadensout the light to strike more of the devices. It also reduces the lensingaction to prevent highly intense light striking the semiconductor layer,which can have the effect of degrading the active layer and so reducesthe effectiveness of the lensing action.

FIG. 3 illustrates complex lensing system 300, showing Fresnel lens 310,carrier case 320, and central coolant channel 330. FIG. 3 depicts anexample of a complex lens design with Fresnel lens 310 on top thatfocuses light through the lensing system onto a concave lens which thenbroadens out the light to strike more of the photovoltaic (PV) devicesand central coolant channel 330.

The stacks are mechanically connected together through a press action(FIG. 4). The stacks are glued together where the glue is a thin film.

FIG. 4A illustrates rod and plate press system 400A, showing stackformation 410A, fastener 420A, rod 430A, and plate 440A. FIG. 4A depictsan example of slides in stack formation 410A where they are physicallybound, but not limited to these methods, by a mechanical press system.

FIG. 4B illustrates rod and bolt press system 400B, stack formation410B, fastener 420B, and rod 430B. FIG. 4B depicts another example ofslides in stack formation 410B where they are physically bound, but notlimited to these methods, by a mechanical press system.

FIG. 5 illustrates an organic photovoltaic (OPV) cell design 500,showing substrate 510, ITO 520, PEDOT:PSS 530, OSC 540, LiF 550, Al 560,and organic photovoltaic cell 570. FIG. 5 depicts a standard organicphotovoltaic (OPV) cell design, vertically oriented and tilted. Thedesign shown in FIG. 5 is contemplated for use without thermal channels.

FIG. 6 illustrates OPV cell design 600 with thermal channels, showingelectrodes 610, first organic active layer 620, thermal channel 630,spacer 640, substrate 650, and second organic active layer 660. FIG. 6depicts tandem organic photovoltaic (OPV) cell design 600 with thermalchannel 630, incorporating 1) electrodes 610, where one electrode is ahigh work function metal and the second electrode is a low work functionmaterial, allowing a Schottky formation, 2) first organic active layer620, 3) thermal channel 630, which may be a the tube flow to take awaythe heat, 4) spacer 640 between two different active layers 620 and 640,5) substrate 650, and 6) second organic active layer 660.

FIG. 7 illustrates inorganic photovoltaic (IPV) cell design 700, showingsubstrate 710, Mo 720, Cu(InGa)Se₂ 730, CdS 740, ZnO 750, ZnO:A 760, andinorganic photovoltaic cell 770. FIG. 7 depicts inorganic photovoltaic(IPV) stacked cell 770 linearly aligned at an angle for optimumabsorption. While not shown here, a semitransparent electrode can alsoin part be used such as Au or Ag. The design shown in FIG. 7 iscontemplated for use without thermal channels.

FIG. 8 illustrates IPV cell design 800 with thermal channels, showingsubstrate 810, Thermal Channel 820, Mo 830, Cu(InGa)Se₂ 840, CdS 850,ZnO 860, ZnO:Al 870, and inorganic photovoltaic cell 880. FIG. 8 depictsa simple Copper Indium Gallium Selenide (CIGS) example of inorganicphotovoltaic (IPV) cells 880 with thermal channels 820.

FIG. 9 illustrates complex cell design formation process 900, showingcell 910, metal electrode 920, semiconductor active layer 930, metalelectrode 940, substrate layer 950, and complex cell design 960. Metalelectrodes 920 and 940 have different workfunctions, except when theactive layer is a pn junction. Within the process, shown by the arrow,of forming complex cell design 960 from cells 910, cell 910 is tiltedvertically so that complex design 960 is oriented vertically. FIG. 9depicts typical cells 910 fabricated in a planar manner where metalelectrode 920 is the top electrode, semiconductor active layer 930 isthe active photovoltaic (PV) layer, metal electrode 940 is the bottomelectrode and substrate layer 950 is the substrate. For organicphotovoltaic (OPV) cells the electrodes have different work functions.For example, Au, AG Pt are examples of high workfunction metals and Al,Ca, Mg are examples of low workfunction metals. Further, alloys may bealso used providing the work function factors are maintained. Forinorganic photovoltaic (IPV) cells, the metal electrodes may be of thesame work function, depending on whether Schottky or p-n type cells areformed. The design shown in FIG. 9 is contemplated for use withoutthermal channels.

FIG. 10 illustrates a tilted complex cell design 1000, showing metalelectrode 1010, semiconductor active layer 1020, metal electrode 1030,substrate layer 1040, light input from the top 1050, and stack 1060.More particularly, FIG. 10 depicts stacks 1060 tilted so that the lightcomes in and is then reflected through the different layers to get toactive photovoltaic (PV) layer 1040. The design shown in FIG. 10 iscontemplated for use without thermal channels.

FIG. 11 illustrates another tilted complex cell design 1100, metalelectrode 1110, semiconductor active layer 1120, metal electrode 1130,substrate layer 1140, and stack 1150. Devices are tilted to allow foroptimum light input and capture. FIG. 11 depicts a design in which metalelectrode 1110 is the top electrode, semiconductor active layer 1120 isthe active photovoltaic (PV) layer, metal electrode 1130 is the bottomelectrode and substrate layer 1140 is the substrate and they 1100, 1120,1130, and 1140 are all vertically stacked perpendicular to the plane buttilted at an angle cp, where the angular dependence is determined by therefractive index of the active SC layer and/or light management. Thedesign shown in FIG. 11 is contemplated for use without thermalchannels.

FIG. 12 illustrates yet another tilted complex cell design 1200, firstcell top tilting 1210, second cell top tilting 1220, and third cell toptilting 1230. The tops of the cells exposed to light are tilted tocapture light in an optimum manner. FIG. 12 depicts exemplaryarrangements 1210, 1220, and 1230, illustrating that the stacks can bearranged in a number of different ways. FIG. 12 shows the stacks priorto the tilting depending on whether a lens system is used). The designshown in FIG. 12 is contemplated for use without thermal channels.

FIG. 13 illustrates a complex cell design 1300 with the thermalchannels, electrodes 1310, first organic active layer 1320, thermalchannel 1330, spacer 1340, substrate 1350, second organic active layer1360, and photovoltaic cell 1370. FIG. 13 depicts tandem design 1300with multiple organic photovoltaic (OPV) cells 1370 and thermal channels1330 (shown non-tilted for diagrammatic reasons).

FIG. 14 illustrates another complex cell design 1400 with thermalchannels, showing electrodes 1410, organic active layer 1420, thermalchannel 1430, substrate 1450, and photovoltaic cell 1460. FIG. 14depicts tandem design 1400 with multiple Inorganic photovoltaic (IPV)cells 1460 and thermal channels 1430 (shown non-tilted for diagrammaticreasons).

Processes for Fabrication of Inorganic Photovoltaic (IPV) Stacked TandemCells:

Processes for fabrication of IPV stacked tandem cells typically involveseven steps.

Step 1. Base

A base is used as the foundation for the fabrication process. Thedimensions of the base typically range from mm² to m², which depends onthe limitation of manufacturing deposition techniques used. The materialfor the base can be, but is not limited to glass, sheet metal or aplastic sheet. The base acts as a structure support for the final stackcells. It can also be peeled away to allow the final stack cells to befree standing.

Step 2. Substrate Deposition

A transparent substrate is deposited on top of the base. The dimensionsof the substrate range from mm² to m² and the thickness of the substratetypically ranges from a few μm to mm, which depends on the limitation ofdeposition techniques used. The material for the substrate can beinorganic such as but is not limited to a thin sheet of amorphoussilicon glass, quartz glass, doped glass or metal oxide. The depositionof the inorganic substrate is achieved with commonly used depositiontechniques for inorganics such as evaporation, sputtering and atomiclayer deposition. Functional dopants can also be added to the substratefor special needs. For example, anti-stokes (IR fluorescent) pigmentscan be used as dopants in the substrate. The pigments are excited by IRenergy, and emit in the visible region at a shorter wavelength at whichthe photoactive polymer layer usually absorbs. Alternatively, UVfluorescent dyes can also be used to convert UV light to the visible ornear IR. Inorganic/organic particles (SiO₂ and metal oxidenanoparticles), fibers (metal oxides nano-fibers and carbon nanotubes)and platelets (clay) can be used for encapsulation, structurereinforcements or as fire retardants.

Step 3. Bottom Contact Deposition

A bottom contact is deposited on top of the substrate. The dimensions ofthe bottom contact typically range from a few mm² to m² and thethickness of the contact typically ranges from a few to several hundrednm. A high work function metal (such as, but not limited to Au, Ag, Co,Ni, Pt, Pd or Mo) can be deposited by evaporation or sputtering. Asemitransparent layer is preferred, although it may not be required forcertain architectures.

Step 4. III-V Semiconductor Light Absorber Deposition

A III-V semiconductor light absorber such as but not limited to CIGS isdeposited on top of the bottom contact. The dimensions of the absorberlayer typically ranges from a few mm² to m² and the thickness of thelayer typically ranges from a few to several μm. An epitaxial growthvapor deposition process is preferred in order to reach high efficiencyof the resulting IPV. The bandgap of semiconductor layer is tuned bychanging the feeding rate of each element during the growth process.

Step 5. Buffer Layer Deposition

A buffer layer such as but not limited to CdS is deposited on top of theabsorber layer. The dimensions of the buffer layer typically ranges frommm² to m² and the thickness of the layer typically ranges from a few toseveral hundred nm. The buffer layer is grown on top of the absorberlayer by chemical-bath deposition (CBD) in order to achieve preferredlattice structures to reduce the charge recombination in thespace-charge region. Alternatively, CdS precursor nanoparticles can bedeposited by “wet” method (spin coating, spray coating, dip coating,doctor blading, inkjet printing and screen printing) followed by in-situsintering to form the final buffer layer.

Step 6. Window Layer Deposition

A window layer such as but not limited to ZnO is deposited on top of thebuffer layer. The dimensions of the window layer typically range from afew mm² to m² and the thickness of the layer typically ranges from a fewto several hundred nm. A bilayer growth of insulating ZnO and conductiveAl₂O₃-doped ZnO by RF sputtering is preferred in order to enhance theeffective diffusion length of minority carriers (electrons). Depositionof ZnO and Al₂O₃-doped ZnO precursor nanoparticles by “wet” method (spincoating, spray coating, dip coating, doctor blading, inkjet printing andscreen printing) followed by sintering in-situ can be an alternative.

Step 7. Top Contact Deposition

A top contact is deposited on top of the window layer. The dimensions ofthe top contact typically range from a few mm² to m² and the thicknessof the contact typically ranges from a few to several hundred nm. A lowwork function metal (Al, Ca or Mg) or metal alloy is deposited byevaporation or sputtering.

Step 8. Repeat Steps 2-7

To fabricate the final IPV stacked cells, repeat step 2 to 7 multipletimes to reach the desired stacking size.

Processes for Fabrication of Organic Photovoltaic Stacked Tandem Cells:

Processes for the fabrication of organic PV stacked tandem cells aredesigned to adopt existing technologies used in the manufacture oforganic PV cell. The whole fabrication process of the present inventioninvolves six steps as shown in FIG. 15.

FIG. 15 illustrates fabrication process 1500, showing base 1510, firstintermediate 1520 formed from base 1510 by a first step involvingsubstrate deposition (10 μm plastic), second intermediate 1530 formedfrom first intermediate 1520 by a second step involving bottom contactdeposition (100 nm Au), third intermediate 1540 formed from secondintermediate 1530 by a third step involving carrier layer deposition (50nm PEDOT:PSS), fourth intermediate 1550 formed from third intermediateby a fourth step involving active polymer layer deposition (active layerA, B, C), fifth intermediate 1560 formed from fourth intermediate 1550by a fifth step involving top contact deposition (100 nm Al), sixthintermediate 1570, formed by a sixth step involving substrate deposition(10 μm plastic), seventh intermediate 1580 formed from sixthintermediate 1570 by a seventh step involving repeating steps 2-6, thatis the second through sixth steps, and organic photovoltaic cell 1590formed from seventh intermediate 1580 by a step involving adding anodeand cathode connections. FIG. 15 depicts a process for fabrication oforganic photovoltaic (OPV) stacked tandem cells 1590.

FIG. 16 illustrates a side view 1600 of the organic PV stacked tandemcells 1590 fabricated in the process of FIG. 15.

Step 1. Base

A base is used as the foundation for the fabrication process. Thedimensions of the base typically range from a few mm² to m², whichdepends on the limitation of deposition techniques used in thefabrication process. The material for the base is but not limited toglass, sheet metal or a plastic sheet. The base acts as a structuresupport for the final stack cells. Alternatively it is also be peeledaway to allow the final stack cells to be free standing.

Step 2. Substrate Deposition

A transparent substrate is deposited on top of the base. The dimensionsof the substrate typically range from a few mm² to m² and the thicknessof the substrate typically ranges from a few μm to mm, which depends onthe limitation of deposition techniques used. The material for thesubstrate can be inorganic such as but not limited to thin sheet ofamorphous silicon glass, quartz glass, doped glass or metal oxide. Thedeposition of the inorganic substrate is achieved with commonly useddeposition techniques for inorganics such as evaporation, sputtering andatomic layer deposition. Alternatively, organic materials can also beused for the substrate, such as, but not limited to vinyl polymers,polyesters, polyimides, polyurethanes, polyureas, cellulose and epoxy.The deposition of the organic substrate is achieved with common polymercommonly used deposition techniques such as spin coating, spray coating,dip coating, doctor blading, inkjet printing, screen printing andevaporation.

In addition, functional dopants can be added to the substrate forspecial needs. For example, anti-stokes (IR fluorescent) pigments can beused as dopants in the substrate. The pigments are excited by IR energy,and emit in the visible region at a shorter wavelength at which thephotoactive polymer layer usually absorbs. Alternatively, UV fluorescentdyes can also be used to convert UV light to the visible or near IR.Inorganic/organic particles (such as SiO₂ and metal oxidenanoparticles), fibers (such as metal oxides nano-fibers and carbonnanotubes) and platelets (clay) can be used for encapsulation, structurereinforcements or as fire retardants.

Step 3. Bottom Contact Deposition

A bottom contact is deposited on top of the substrate. The dimensions ofthe bottom contact typically range from a few mm² to m² and thethickness of the contact typically ranges from a few to several hundrednm. A semitransparent high work function metal (such as, but not limitedto Au, Ag, Co, Ni, Pt, Pd or Mo), metal oxide (such as ITO) or organicconductor (such as, but not limited to carbon nanotube, graphene,polymers) can be deposited by evaporation, sputtering, or inkjetprinting of metal nanoparticles followed by annealing.

Step 4. Carrier Layer Deposition

A carrier layer is deposited on top of the bottom contact. Thedimensions of the carrier layer typically range from a few mm² to m² andthe thickness of the contact typically ranges from a few to severalhundred nm. For conventional organic solar cell architectures, a chargecarrier layer (PEDOT:PSS) is deposited. For an inverted architecture, anelectron carrier layer (zinc oxide nanoparticle or nanorods) isdeposited. Deposition of the carrier layer is achieved by common polymerdeposition techniques such as spin coating, spray coating, dip coating,doctor blading, inkjet printing, screen printing. An annealing step maybe needed for improved carrier conduction.

Step 5. Active Polymer Layer Deposition

A photoactive polymer layer or tandem layers are deposited on top of thecarrier layer. The dimensions of the carrier layer typically range froma few mm² to m² and the thickness of the contact can range from a few toseveral hundred nm. Potential active components for organic photovoltaicstacked tandem cells are the following: P3HT/PCBM blends (spectrumcoverage 400-600 nm), Cu₂S quantum dots/CdS nanorods (600-800 nm), andPbS or PbSe quantum dots (800-1200 nm, polymers that may be used arepolythiophenes and other polymer conjugated systems with suitablebandgap, spectral and transport conditions such as P3HT or P3OT. Thedesign of the present invention allows for the use of any appropriatematerial whose absorption spectrum matches that of the solar spectrum.The deposition of single photoactive polymer layer is achieved withcommonly used polymer deposition techniques such as spin coating, spraycoating, dip coating, doctor blading, inkjet printing, screen printing.The deposition of two or more photoactive polymer layers is achieved byinkjet printing and screen printing. An annealing step may be needed toform optimized bulk heterojunctions for improved solar cellefficiencies.

Step 6. Top Contact Deposition

A top contact is deposited on top of the active layer. The dimensions ofthe top contact typically range from a few mm² to m² and the thicknessof the contact typically ranges from a few to several hundred nm. A lowwork function metal (such as, but not limited to Al, Ca or Mg), metalalloy or MgIn alloy is deposited by evaporation or sputtering. Forreverse or inverted architecture, a high work function metal (such as,but not limited to Au, Ag, Co, Ni, Pt, Pd or Mo), metal oxide or organicconductor (such as, but not limited to carbon nanotube, graphene,polymers) is deposited by evaporation, sputtering, or inkjet printing ofmetal nanoparticles followed by annealing. An annealing step may beneeded to form optimized polymer/contact interfaces for improved solarcell efficiencies.

Step 7. Repeat Steps 2-6

To fabricate the final OPV stacked cells (FIG. 16), repeat step 2 to 6multiple times to reach the desired stacking size.

Dependence of Efficiency on Angle of Entry of Incident Light

In order to maximize absorption of light by the active layer, light mustenter the device at a specific optimum angle (θ₀), which is determinedby the respective layer thicknesses. Light will be absorbed to a fargreater extent using this ‘trapping’ geometry due to the maximized pathlength that the incident light has to travel through the active layer.This principle is applied to an array of stacked cells which areorientated so that normally incident light from the sun will actuallyenter the stacked array at the optimum angle.

FIG. 17 is a simplified ray diagram, illustrating light passing throughstacked photovoltaic cells, showing incident light 1710, incidence angleθ₀, cathode 1720, substrate 1730, anode 1740, carrier layer 1750,photoactive layer 1760, mirrored surface 1770, 1st Stack Layer 1780, and2nd Stack Layer 1790. Incident light 1710 may be sunlight. Itillustrates the path that incident light travels through the device.Once the unabsorbed light reaches the bottom of each stack 1780 and1790, it is incident upon a mirrored surface. It is then reflected backalong each respective stack 1780 and 1790, to be further absorbed. Onits second pass along each respective stack 1780 and 1790, it passesthrough active layer 1760 again multiple times. This light trappingprocess is forcing the incident light to travel through effectively afar ‘thicker’ photoactive layer than possible with conventional flatpanel architectures. Such configuration addresses the loss of lightinherent with the flat panel.

In order to convert more of the incident light energy, it is necessaryas mentioned previously, to use more than one photoactive layer. Aslight modification on the stack allows for the use of two (twin—asshown in FIG. 18), three (tri-) or even four (quad-) or more cellphotoactive layer architectures.

FIG. 18 is a simplified ray diagram illustrating light passing throughstacked twin photovoltaic (PV) cells, showing incident light 1810, angleθ₀, cathode 1820, substrate 1830, anode 1840, carrier layer 1850, twinphotoactive layer 1860, mirrored Surface 1870, 1st stack layer 1880, and2nd stack layer 1890. Twin photoactive layer 1060 contains a firstphotoactive layer, a spacer, and a second photoactive layer. Incidentlight 1810 may be sunlight. FIG. 18 illustrates the path that incidentlight travels through the ‘twin’-active layer material device. Once theunabsorbed light reaches the bottom of each stack 1880 and 1890, it isincident upon minored surface 1870. It is then reflected back along eachrespective stack 1880 and 1890, to be further absorbed. On its secondpass along each respective stack 1880 and 1890, it passes through theactive layer 1860 again multiple times.

Stack Contacting

1. For Stacking of Photovoltaic Cells Made of Glass or InflexibleSubstrate:

Two metal strips can be deposited a shown in FIG. 19A. FIG. 19Aillustrates metal strips deposited on a substrate with photovoltaic (PV)devices, showing photovoltaic cell 1900A, glass 1910, metal strips 1920,cathodes (e.g. aluminium) 1930, photoactive layer 1940, anode (e.g.indium tin oxide) 1950. Metal strip 1020 connects cathodes 1930. The twometal strips make a parallel connection of the PV devices. One stripconnects the cathodes (e.g. Al) of the PV devices. The other strip isdeposited partly superimposing the anode (e.g. ITO). The deposition isalso done on the sides of the substrate to extend the cathode and theanode.

The substrate can then be stacked as shown in FIG. 19B. FIG. 19B shows aside view of stack 1900B of the photovoltaic (PV) cells of FIG. 19A,showing glass 1910 and metal strip 1920.

To connect the electrodes of all the glass slides, a conductive tape canbe used in the manner as shown in FIG. 19C. FIG. 19C illustratesconductive tapes applied on the exposed metal strips shown in FIG. 19B,showing taped stack 1900C, glass 1910, metal strips 1920, and conductivetapes 1960.

The conductive tape can be: A metal tape stuck to the side of the stackusing conductive epoxy. Alternatively, a conductive nanoparticle ink ormetal paste, which upon heating gives a uniform layer can be used.

2. For Stacking PV Cells Made of Flexible Substrates:

Two metal strips are deposited. One strip connects the cathodes of thepixel devices. The other is deposited covering the anode partly (FIG.19D). FIG. 19D illustrates metals strips 1920 deposited on a flexiblesubstrate 1915 with photovoltaic (PV) devices, showing photovoltaic cell1900D, flexible substrate 1915, metal strips 1920, cathodes 1930,photoactive layer 1940, and anode 1950. The flexible substrate is bentat the side without damaging the metal strips (FIG. 19E).

FIG. 19E illustrates bending of flexible substrate 1915 without damagingthe connections, showing bent photovoltaic cell 1900E, flexiblesubstrate 1915, metal strips 1920, cathodes 1930, photoactive layer1940, and anode 1950.

When such substrates are stacked on top of one another it will have astructure as shown in FIG. 19F. FIG. 19F illustrates stacking of thephotovoltaic (PV) devices shown in FIG. 19E, showing stack 1900F,flexible substrate 1915, metal strips 1920, cathodes 1930, photoactivelayer 1940, and anode 1950. A part of each of the metals stripsconnecting the cathode and anode of the pixel devices on each substrateis exposed.

A conductive tape (which has been mentioned earlier) is used to connectthe exposed part of the metal strips as shown in FIG. 19G. FIG. 19Gillustrates conductive tapes 1960 applied on exposed metal strips 1920shown in FIG. 19F, showing taped stack 1900G, flexible substrate 1915,metal strips 1920, cathodes 1930, photoactive layer 1940, and anode1950.

Thermal Solar Cells

The device described herein also addresses a critical problem that iswell-known of photovoltaic devices, which is the thermal degradation ofthe cell components due to temperature building caused by exposure tothe sun. Over time, thermal degradation can become a big problem thataffects the longevity of cells. The present invention addresses thatissue by having thermal fluid inlets on the base substrate, asillustrated in FIGS. 2B, 3, 6, 8, 13, and 14, that collects anddissipates the heat away from the other cell components. This can alsobe achieved using the concentrator architecture where the light ischanneled into the cells that contain thermal fluid channels (using atransparent fluid such as water) to absorb and hence reduce the thermalenergy generation. The light concentration can be achieved by usingfresnel lens, parabolic mirrors or derivatives of such structures (asshown in FIGS. 2B and 3). The contacting can be done in series orparallel, and examples of the contacting shown in FIGS. 19E, 19F, and19G).

What is claimed is:
 1. A photovoltaic device adapted for receivingincident light, comprising: a plurality of photovoltaic cells, whereineach of the plurality of photovoltaic cells comprises: a firstconductive layer, a photoactive layer adjacent the first conductivelayer, wherein the photoactive layer comprises an organic material, asecond conductive layer adjacent the photoactive layer, a substrateadjacent to one of the first conductive layer or second conductivelayer, a first planar interface is formed between the first conductivelayer and the photoactive layer, and a second planar interface is formedbetween the photoactive layer and the second conductive layer; whereineach layer of the plurality of photovoltaic cells has an upper surfaceand a lower surface, the lower surface is opposite to the upper surface,and each layer of the plurality of photovoltaic cells are stackedparallel to each other in a stacking direction; wherein a first top endof the plurality of photovoltaic cells nearer the incident light forreceiving the incident light is a first plane formed by the uppersurface of each of the first conductive layer, the photoactive layer,the second conductive layer, and the substrate of the plurality ofphotovoltaic cells, and a second bottom end of the plurality ofphotovoltaic cells is a second plane formed by the lower surface of eachof the first conductive layer, the photoactive layer, and the secondconductive layer of the plurality of photovoltaic cells; wherein thefirst planar interface and the second planar interface of each of theplurality of photovoltaic cells are oriented perpendicular to both thefirst plane of the first top end and the second plane of the secondbottom end of the plurality of photovoltaic cells; wherein the first topend of the plurality of photovoltaic cells is spaced apart and parallelto the second bottom end of the plurality of photovoltaic cells in avertical direction perpendicular to the stacking direction; wherein achannel runs through the substrate and is oriented to extendperpendicular to both the stacking direction and the vertical direction;and a reflective surface in contact with the second bottom end of theplurality of photovoltaic cells.
 2. The photovoltaic device according toclaim 1, wherein one or more of the first conductive layer and secondconductive layer comprises a material selected from among transparentand semi-transparent materials.
 3. The photovoltaic device according toclaim 1, wherein the photoactive layer comprises a thin film.
 4. Thephotovoltaic device according to claim 1, wherein each of the pluralityof photovoltaic cells further comprises a fully-reflective layer orsemi-reflective layer.
 5. The photovoltaic device according to claim 1further comprising a lensing system disposed between the incident lightand the first top end of the plurality of photovoltaic cells.